Dynamic mask projection stereo micro lithography

ABSTRACT

Three-dimensional structures are fabricated by a process in which a 3D solid model is designed by software at a PC and sliced into a series of 2D layers. Each 2D layer is displayed at a dynamic mask via micro-mirror deflections projected onto a photoresist to form a layer, which is lowered and the process is repeated to build the object layer by layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on U.S. Application Ser. No. 60/235,799, filedSep. 27,2000.

FIELD OF THE INVENTION

The field of the invention is microfabrication of 3D structures

BACKGROUND OF THE INVENTION

Micro-electro-mechanical systems (MEMS) devices have been found in manysensing applications such as airbag sensors, as well as chemical andbiological sensors. The use of micro actuators is the key to making MEMSfully active, intelligent “micro-system” devices, capable of bothsensing and actuating. Current IC-based micromachining processes, usedto fabricate MEMS devices, have certain limitations in achieving theabove goals. First, most of the IC-based micromachining processes cannotbe used to fabricate complex 3D micro parts with high aspect ratios.Second, only a few semiconductors and other materials can be processedby the current IC-based micromachining for MEMS. Many other importantengineering materials, such as smart ceramics, functional polymers, andmetal alloys, can not be directly incorporated into MEMS through theconventional IC-based micromachining processes.

As an alternative, an x-ray LIGA (German Lithography, electroforming andmolding) process was developed to fabricate microstructures with highaspect ratio. However, the x-ray LIGA process has not found a largenumber of industrial applications due to its poor industrialaccessibility and operational cost. In addition, complex 3D structurescan not be achieved by an LIGA process. A novel microfabricationprocess, the micro stereolithography (μSL) was introduced to fabricatehigh aspect ratio and complex 3D microstructure by single beam scanning.[Ikuta, K., Ogata, T., and Kojima, S., 1996, “Development of massproductive micro stereo lithography”, Proc. IEEE MEMS'96, pp. 301–305.]

Sophisticated 3D parts can be made by scanning an UV beam on a liquidmonomer resin, curing the resin into solid polymer layer by layer, andstacking together all layers with various contours.

Recently, liquid crystal display projection micro-stereolithography hasbeen used by groups in Europe to project a layer image onto thephoto-resist. However, due to the large pixel size and very lowtransmission in UV, the device's resolution is limited and contrast ispoor. Macro scale stereo-lithography, on the other hand, has been aroundfor more than 15 years, mainly with single beam scanning, and commercialapplications in this area have also been established such as by 3DSystems Inc. Only recently, scientists have started to use digital lightprocessing technology, for example, using a commercial digitalmicro-mirror display projector from Texas Instrument to do macro-scalestereo-lithography. However, they use mainly a commercial optical setupfrom Texas Instruments and focus on macro-scale fabrication forstructure and automotive applications rapid prototyping. See, also, U.S. Pat. No. 6,200,646 to Neckers et al, entitled: “Method For FormingPolymeric Patterns, Relief Images And Colored Polymeric Bodies UsingDigital Light Processing Technology.”

SUMMARY OF THE INVENTION

The present invention uses micro stereolithography to provide a newmethod to fabricate 3D micro or nano structures that can be used for awide variety of devices such as micro/nano-electronics, biotechnology,MEMS, biomedical devices and in the manufacture of optical devices suchas lenses and mirrors. The invention is based on using advanced dynamicmask projection stereo micro-lithography on a photoresist to form alayer, building an object layer by layer, to achieve ceramicmicro-stereolithography for the first time. A 3D solid image, which maybe a model designed by CAD software at a PC, is sliced into a series of2D layers, each 2D layer being displayed at the dynamic mask viamicro-mirror deflections projected onto the photoresist.

Using individually addressed micro mirrors, gray tone images can beachieved and 3D object can be formed by one exposure. The gray tone ofthe micro mirrors also provides a unique method to compensate for lightfield aberrations due to optical elements such as lenses and apertures,ensuring a uniform light field with ordinary optics. This is especiallyuseful for future aberration-free optical engineering and lithography inmicron or sub-micron electronics.

More particularly, a method and apparatus are provided for forming athree-dimensional structure from a computer-stored image in which one ofa plurality of sequential two-dimensional slices of a computer storedimage is used to control a digital micro-mirror display arranged toreflect actinic light to a photo-curable composition as an image of theselected slice. The reflected light image is focused to a first locationin the composition to cure a two-dimensional layer therein correspondingto said selected two-dimensional slice. The process is repeated withsequentially selected slices focused to sequential locations in thecomposition until the three-dimensional structure is formed.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood. Theforegoing, and additional features and advantages of the invention willbe described hereinafter, form the subject of the claims of theinvention. It should be appreciated by those skilled in the art that theconception and specific embodiment disclosed may be readily utilized asa basis for modifying or designing other structures for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims. The novel features which are believed to becharacteristic of the invention, both as to its organization and methodof operation, together with further objects and advantages will bebetter understood from the following description when considered inconnection with the accompanying figures. It is to be expresslyunderstood, however, that each of the figures is provided for thepurpose of illustration and description only and is not intended as adefinition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS AND TABLES

FIG. 1 is a schematic depiction of the method of this invention.

DETAILED DESCRIPTION

The present invention is used to fabricate 3D complex micro/nanostructures by digital light processing using a dynamic mask. Digitallight processing technology is a high brightness and contrast, highresolution technique composed of a micro-electro-mechanicalsemiconductor based array of small, fast independently hinged reflectivesurfaces (or mirrors) the combination of which is known as a digitalmicro-mirror device or display (DMD). The reflective surfaces, each ofwhich represents a single pixel, can be of very small size, in themicrometer range, or even smaller. In a non-limiting embodiment,reflective surfaces are in the range of 0.1–17 μm. They are squaremirrors that can reflect light in two directions, namely “on” and “off.”In the on position, the mirrors reflect light through the optics onto orinto the object being exposed and the mirrors appear bright. Conversely,when the mirrors are in the off position, the light is reflected out ofthe optical field and the mirrors appear dark. Thus when a DMD iscombined with a light source, an image processor, and optics, it can beutilized to precisely control the pattern of the light source using thisbinary pulse width modulation technique.

A DMD can also control the intensity of the pattern pixels. Using binarypulse width modulation or other signaling techniques, the amount of timethe mirrors are in the off and on positions can be precisely controlled.By quickly switching a particular mirror between the on and offpositions or pulsing the mirror, the effective intensity of light at theobject surface is controlled. When the mirror is continuously on(corresponding to a white pixel) the effective intensity at the objectsurface is high. However, if the mirror is modulated between the on andoff state, the effective intensity can be precisely controlled based onthe amount of time the mirror is in the on position. For example, usingknown digital light processing technology, a single DMD system cangenerate 256 gray levels.

An example of a useful dynamic mask is a digital mirror based displaydevice sold commercially by Texas Instruments and which has 17 μmmirrors, micro-second response time, and high contrast and definitioncompared with other display devices such as liquid crystal devices.Other programmable arrays which operate in a fashion similar to theTexas Instruments DMD may be utilized.

Referring to FIG. 1, a 3D solid model is designed by any suitablemethod, e.g., by bit mapping or by computer aided design (CAD) softwareat a PC/controller 10. The CAD model is then electronically sliced intoseries of 2-dimensional data files, ,i.e., 2D layers, each defining aplanar cross section through the device to be constructed, and which maybe individually stored. Alternatively, the slices may be applieddirectly in real time. In either event, each 2D layer data is used tocontrol a DMD display via the PC 10. A beam shutter 12, which may be anelectronic or mechanical shutter, or any other type, is controlled bythe PC 10 and in turn controls a light beam 14 which then travelsthrough a beam homogenizer 16 and a narrow band filter 18, impinging onthe mirror 20 of a prism 22 to reflect therefrom to a DMD chip 24.

The DMD chip 24 is formed with more than a half million mirrors, eachindividually addressed electronically and deflecting a homogenized lightbeam back through the prism 22. Actinic wavelengths of the image pass abeam splitter 38 and is then reflected by a mirror 28 to a beamconditioning system 30 which consists of appropriate apertures andlenses, and then to and through a reduction projection system 32 (whichis available commercially from lithography tool companies). Theprojected image is in focus at a particular plane in aphotoresist/photo-curable composition 34 in a supporting container 36 sothat the actinic light preferentially exposes the desired layer to cureit.

A source of actinic light 11 may be any number of available sources,including, but not limited to metal halide, tungsten halogen and xenonlamps or lasers, e.g., an ultraviolet light laser. A beam splitter 38 isused to assist monitoring/alignment for the fabrication. At thebeginning or during each layer fabrication, the same light imaged on theresin surface can be reflected back through the same projection optics32, beam conditioning system 30, mirror 28, and finally reflected by thebeam splitter 38 into a monitoring system 40 which consists of a CCD andother optical accessories.

After one layer is cured, or at least partially cured sufficient toretain its integrity, the composition and focusing optics undergorelative movement. In a particular embodiment of the invention, acomputer controlled z elevator stage 42 that carries the cured layermoves down and a second layer of fresh photoresist is formed to beexposed. The layer-by-layer process continues until a 3D object isfabricated. The elevator motion, beam on/off, and micro-mirror displayare synchronized and controlled by the computer 10.

The photo resist, i.e., photo-curable composition, can be materialobtained using conventional techniques in photochemistry andphotopolymer chemistry which are familiar to those skilled in the art.The photoimaging and polymerization steps may employ a liquid,semi-solid or solid photopolymerizable composition which is cured byactinic radiation such as ultraviolet, visible or infrared irradiation,depending on the photopolymer. Specific examples include HDDA (the1,6-hexanediol diacrylate) and other commercially available (from Ciba)stereolithography acrylate (SL 5149) and hybrid epoxy-acrylate resins(SL 5170) with light initiators, such as BEE (Benzoin ethyl ether),added. Other monomers, photoinitiators, dopant and other components thatcan be used will be apparent to one skilled in the art. In a significantembodiment of the invention, dopant is added to the photo-curablecomposition. This is very useful in controlling the penetration depth ofeach 2D layer, enabling the fabrication of truly 3D micro or nano scalestructures, especially for overhang structures. The dopant is a lightabsorbing chemical such as Tinuvin (by Ciba Geigy Corp.) dispersed inthe photo-curable resin or composition. Amounts of dopant from 0.1 to 5%can generally be used. In general, monomers or oligomers or mixturesthereof are used, and may be composed of acrylates, including mono andmultifunctional acrylates and methacrylates; epoxides such asepoxycyclohexane derivatives; or hybrid mixtures of both acrylate andepoxides. Such compositions contain appropriate photoinitiators suchthat light of the desired wavelength activates the initiation of thepolymerization reaction.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and/or steps described in the specification.As one of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

1. A method for forming a three-dimensional structure from acomputer-stored image, comprising the steps of: (a) providing aphoto-curable composition; (b) providing a source of actinic light forcuring said composition; (c) arranging a digital micro-mirror display toreflect said actinic light to said composition; (d) providing a computerstored image; (e) computer-generating a plurality of sequentialtwo-dimensional slices from said image; (f) using said plurality ofsequential two-dimensional slices to sequentially control said digitalmicro-mirror display to reflect said actinic light to said compositionas sequential images of said selected slice; (g) passing said reflectedlight images through a reduction projection system to enhanceresolution; and (h) sequentially focusing said reflected light images toa plurality of successive locations in said composition to at leastpartially cure successive two-dimensional layers therein correspondingto said sequential two-dimensional slices until the three-dimensionalstructure is formed; wherein a portion of said reflected light is usedto view the composition as said three-dimensional structure is beingformed.
 2. The method of claim 1 in which said computer-stored image isa solid 3D model.
 3. The method of claim 2 in which said model is a CADimage.
 4. The method of claim 1 in which said sequential locations arevertical locations.
 5. The method of claim 4 in which actinic lightreflected from said digital micro-mirror display is focused by optics toa predetermined vertical location, and wherein sequential locations insaid composition are obtained by relative vertical movement of saidcomposition and said optics.
 6. The method of claim 5 in whichsequential locations in said composition are obtained by sequentialdownward vertical movement of said composition.
 7. The method of claim 1in which a light absorbing chemical dopant is dispersed in thephoto-curable composition, for precisely controlling the curing depth.8. An apparatus for forming a three-dimensional structure in aphoto-curable composition from a computer-stored image, comprising: asource of actinic light for curing said composition; a digitalmicro-mirror display arranged to reflect said actinic light to saidcomposition; a computer for providing a computer stored image andgenerating a plurality of sequential two-dimensional slices from saidimage; means for using said sequential two-dimensional slices to controlsaid digital micro-mirror display to sequentially reflect said actiniclight to said composition as a sequence of images of said slices; meansfor focusing and enhancing resolution of said reflected light imagessequentially to a plurality of locations in said composition to at leastpartially cure successive two-dimensional layers therein correspondingto said sequential two-dimensional slices whereby to enable thethree-dimensional structure to be formed; and means for viewing aportion of said reflected light whereby to enable viewing of thecomposition as said three-dimensional structure is being formed.
 9. Theapparatus of claim 8 in which said computer-stored image is a solid 3Dmodel.
 10. The apparatus of claim 9 in which said model is a CAD image.11. The apparatus of claim 8 in which said sequential locations arevertical locations.
 12. The apparatus of claim 11 in which the means forfocusing and enhancing resolution of said reflected light imagescomprises optics focusing said actinic light reflected from said digitalmicro-mirror display to a predetermined vertical location, and means forrelative vertical movement of said composition and said optics.
 13. Theapparatus of claim 12 in which said means for relative vertical movementof said composition and said optics comprises means for sequentialdownward vertical movement of said composition.
 14. The apparatus ofclaim 13 in which said means for sequential downward vertical movementof said composition comprises a computer controlled z elevator stagecarrying the composition.